Highly Ordered Single Crystalline Nanowire Array Assembled Three

Nov 18, 2015 - Highly Ordered Single Crystalline Nanowire Array Assembled Three-Dimensional Nb3O7(OH) and Nb2O5 Superstructures for Energy Storage and...
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Highly Ordered Single Crystalline Nanowire Array Assembled Three-Dimensional Nb3O7(OH) and Nb2O5 Superstructures for Energy Storage and Conversion Applications Haimin Zhang,† Yun Wang,§ Porun Liu,§ Shu Lei Chou,‡ Jia Zhao Wang,‡ Hongwei Liu,ξ Guozhong Wang,† and Huijun Zhao*,†,§ †

Key Laboratory of Materials Physics, Centre for Environmental and Energy Nanomaterials, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China § Centre for Clean Environment and Energy, Griffith University, Gold Coast Campus, QLD 4222, Australia ‡ Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia ξ Australian Centre for Microscopy & Microanalysis, The University of Sydney, NSW 2006, Australia S Supporting Information *

ABSTRACT: Three-dimensional (3D) metal oxide superstructures have demonstrated great potentials for structure-dependent energy storage and conversion applications. Here, we reported a facile hydrothermal method for direct growth of highly ordered single crystalline nanowire array assembled 3D orthorhombic Nb3O7(OH) superstructures and their subsequent thermal transformation into monoclinic Nb2O5 with well preserved 3D nanowire superstructures. The performance of resultant 3D Nb3O7(OH) and Nb2O5 superstructures differed remarkably when used for energy conversion and storage applications. The thermally converted Nb2O5 superstructures as anode material of lithium-ion batteries (LiBs) showed higher capacity and excellent cycling stability compared to the Nb3O7(OH) superstructures, while directly hydrothermal grown Nb3O7(OH) nanowire superstructure film on FTO substrate as photoanode of dye-sensitized solar cells (DSSCs) without the need for further calcination exhibited an overall light conversion efficiency of 6.38%, higher than that (5.87%) of DSSCs made from the thermally converted Nb2O5 film. The high energy application performance of the niobium-based nanowire superstructures with different chemical compositions can be attributed to their large surface area, superior electron transport property, and high light utilization efficiency resulting from a 3D superstructure, high crystallinity, and large sizes. The formation process of 3D nanowire superstructures before and after thermal treatment was investigated and discussed based on our theoretical and experimental results. KEYWORDS: Nb3O7(OH), Nb2O5, 3D nanowire superstructures, lithium-ion batteries, DSSCs

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superstructures could pave a way for development of high performance energy materials. As compared to widely investigated energy materials (e.g., TiO2), niobium oxides including NbO, NbO2, Nb2O3, Nb2O5, and Nb3O7X (X = OH, F) have demonstrated superior performance in catalysis, photocatalysis, lithium-ion batteries (LiBs), sensors, and solar cells.14−21 The reported studies have demonstrated that monoclinic Nb2O5 is a promising energy material to improve the open-circuit voltage of dye-sensitized

hree-dimensional (3D) metal oxide superstructures have aroused great attention in the areas of clean environment and energy because of their large surface area and unique photonic, electronic, and optical properties, differing significantly from individual entities of superstructures, suitable for structure-dependent applications.1−4 To fully utilize superstructure properties for structure-dependent applications, varieties of metal oxides with different 3D structures, such as ZnO,4 Co3O4,5 V2O5,6 SnO2,7 and α-Fe2O38 hollow microspheres, manganese oxides,9 and TiO210−12 superstructures have been extensively investigated for energy applications, exhibiting significantly improved energy storage and conversion performances.13 Therefore, exploring 3D complex metal oxide © 2015 American Chemical Society

Received: August 30, 2015 Accepted: November 18, 2015 Published: November 18, 2015 507

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ACS Nano solar cells (DSSCs) and energy density of LiBs, because of its negative conduction band edge position and large lithium ion insertion number of 2.5 (x = 2.5 for LixNb2O5).17,22 However, all reported Nb2O5 nanostructures such as nanoparticles and nanobelts obtained by traditional sol−gel and electrospinning methods (requiring high temperature calcination treatment at 1000 °C) usually possess low surface area, leading to unsatisfactory application performance.17,18,23 Recently, orthorhombic Nb3O7(OH) complex nanostructures have been synthesized and successfully demonstrated for environmental and energy applications.20,24−26 Importantly, our recent study confirmed that the orthorhombic Nb3O7(OH) nanorods can be readily transformed into monoclinic phase Nb2O5 crystal structure while the morphology and high surface area are well preserved.24 This inspired us to utilize the same approach to obtain monoclinic Nb2O5 superstructures from orthorhombic Nb3O7(OH) superstructures, enabling us to meaningfully investigate the benefits of 3D superstructures and structuredependent performance in energy applications. To the best of our knowledge, a meaningful comparison of energy storage and conversion performance of Nb3O7(OH) and Nb2O5 with similar 3D superstructures has not been reported. Herein, we present a template-free synthesis of 3D orthorhombic Nb3O7(OH) superstructures assembled with a highly ordered single crystalline nanowire array and their lowtemperature thermal transformation (at 450 °C) into monoclinic Nb2O5 with well preserved 3D nanowire superstructures. The as-synthesized Nb3O7(OH) nanowire superstructures possess a bimodal-pore structure (micro- and mesopores) and a high surface area of 121.5 m2 g−1, while the resultant monoclinic Nb2O5 nanowire superstructures possess the same bimodal-pore structure and a surface area of 81.8 m2 g−1. Compared to the 3D Nb3O7(OH) nanowire superstructures, the thermally converted 3D Nb2O5 nanowire superstructures manifest higher capacity and good cycling stability when evaluated as anode material for LiBs, while the as-synthesized 3D Nb3O7(OH) nanowire superstructure film directly grown onto a FTO conductive substrate as a DSSC photoanode exhibits an overall light conversion efficiency of 6.38%, higher than that of DSSCs constructed by 3D Nb2O5 nanowire superstructure photoanode. The 3D Nb3O7(OH) nanowire superstructure formation, thermal transformation, and the accompanied composition changes were investigated and discussed on the basis of our theoretical and experimental results.

Figure 1. (A) XRD patterns of as-synthesized sample obtained at 200 °C for 24 h and calcined sample at 450 °C for 2 h. (B) Surface SEM image of as-synthesized Nb3O7(OH) sample. (C) SEM image of an individual Nb3O7(OH) structure. (D) TEM image of an individual Nb3O7(OH) structure; the arrows in images C and D denote the preferential growth orientation of single crystalline nanowires. (E) TEM image of an individual Nb3O7(OH) nanowire. (F) FFT image. (G) Index of the FFT image F. (H) Inverse FFT image from the marked area in image E.

can be further validated by the TEM sample obtained from strong ultrasonic treatment (Figure S1, Supporting Information). The TEM image of an individual Nb3O7(OH) nanowire is given in Figure 1E, confirming a preferred nanowire growth along the [010] direction. Figure 1F is the fast Fourier transformation (FFT) image corresponding to the marked area in Figure 1E, indicating a good crystalline nature of the nanowire. The diffraction pattern in Figure 1E can be indexed to (200) and (110) crystal planes of the orthorhombic Nb3O7(OH) (Figure 1G).24 The inverse FFT image further confirms that the fringe spacing is 1.04 nm, corresponding to a d value of (200) plane for the orthorhombic Nb3O7(OH) (Figure 1H).24 After calcination, the superstructures assembled with the highly ordered nanowire array can be well preserved (Figure S2A and B, Supporting Information). Compared to the Nb3O7(OH) nanowire superstructures, the calcined sample displays bundled nanowire assembly with a more compact arrangement as shown by the corresponding TEM images (Figure S2C and D, Supporting Information). Further, the SAED pattern and HRTEM image of an individual nanowire after calcination reveal good crystallinity. The nanowire fringe spacings are 0.375 and 1.02 nm, corresponding to the d values of (110) and (101) crystal planes of monoclinic Nb2O5, respectively.24 In our previous work, the theoretical and

RESULTS AND DISCUSSION Figure 1A shows the X-ray diffraction (XRD) patterns of the assynthesized and calcined samples. The XRD data reveal that the as-synthesized sample is an orthorhombic Nb3O7(OH) structure (JCPDS, Card No. 31-0928), similar to previous reports.24,27 After calcination, the orthorhombic Nb3O7(OH) structure is transformed into the monoclinic Nb2O5 structure (JCPDS, Card No. 71-0005).24 The scanning electron microscopy (SEM) image shown in Figure 1B indicates that the as-synthesized Nb3O7(OH) structures have an average size of 2.6 ± 0.9 μm. Interestingly, the obtained Nb3O7(OH) superstructures are found to be composed of a cubic-shaped core with highly ordered nanowire arrays vertically grown on each cubic surface (Figure 1C). The average nanowire diameter is ∼17.6 nm. The nanowire array constructed superstructure with preferential growth orientation was also confirmed by transmission electron microscopy (TEM) (Figure 1D). This 508

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Figure 2. (A) XRD patterns of as-synthesized samples obtained at 200 °C with different hydrothermal reaction times and their corresponding SEM images. (B) Atomic structure model of cubic HNbO3. (C) Atomic structure models of HNbO3 and Nb3O7(OH), and the calculated thermodynamic reaction energy. (D) Atomic structure models of HNbO3 (100) plane and Nb3O7(OH) (010) plane. Atom color code: green, Nb; red, O; pink, H; green octahedron, [NbO6].

confirm that the 3D Nb3O7(OH) superstructures possess a large surface area of 121.5 m2 g−1 (Figure S5A, Supporting Information). After calcination, the monoclinic Nb2O5 nanowire superstructures show a surface area of 81.8 m2 g−1 (Figure S5A, Supporting Information). Although the surface area of the calcined sample decreases, it is still significantly higher than those of the monoclinic Nb2O5 nanostructures obtained by sol−gel and electrospinning methods, benefiting from the relatively low calcination temperature and unique 3D superstructure.18,23 The pore size analysis indicates that Nb3O7(OH) and Nb2O5 samples both display bimodal-pore structure characteristics, namely, micropores and mesopores. As shown, narrow pore size distributions in microporous scope of Nb3O7(OH) and Nb2O5 samples are centered around 1.3 and 1.1 nm, respectively (Figure S5B, Supporting Information), which is responsible for their high specific surface areas. Also, wide pore size distributions in the mesoporous range centered at 19.8 and 25.3 nm are observed from the Nb3O7(OH) and Nb2O5 samples, respectively, which is also an important attribute to the high surface area. More importantly, the mesoporous structures of Nb3O7(OH) and Nb2O5 samples are favorable for electrolyte transport when used as energy materials for LiBs and DSSCs. Therefore, the unique bimodal-pore structure characteristics of the 3D superstructures could be beneficial for improving the performance of energy applications. To understand the progressive structural formation process of the 3D Nb3O7(OH) superstructures assembled with the highly ordered single crystalline nanowire array, the effect of hydrothermal reaction time on the crystal structure and morphology formation was investigated experimentally. Figure 2A shows the XRD patterns of the products obtained from different reaction times and their corresponding SEM images. The XRD results indicate that the obtained products with short reaction times (e.g., < 9 h) possess different crystal structures as compared to those obtained from longer reaction times (e.g., > 9 h). When the hydrothermal treatment time is less than 9 h, the as-synthesized samples possess a cubic HNbO3 structure with lattice parameters of a = b = c = 7.645 Å (JCPDS, Card No. 36-0794) (Figure 2B). Moreover, the diffraction peak intensity of the (100) plane of the cubic HNbO3 structure

experimental results have indicated that the OH groups in the (002) plane of orthorhombic Nb3O7(OH) can be readily removed through a dehydration process during low-temperature calcination, leading to a serious crystal structure deformation of the Nb3O7(OH) along the z axis direction to form the monoclinic Nb2O5.24 This allows easy transformation of the orthorhombic Nb3O7(OH) into the monoclinic Nb2O5 via low-temperature calcination while maintaining the original superstructures and single crystal nature of the individual nanowire. An important advantage of such a low-temperature calcination approach is that the obtained monoclinic Nb2O5 retains not only 3D nanowire superstructures but also high surface area, favorable for energy storage and conversion applications. The as-synthesized Nb3O7(OH) and thermally converted monoclinic Nb2O5 were further characterized by the X-ray photoelectron spectroscopy (XPS) technique. The XPS survey spectra show that no significant difference can be observed from these two samples (Figure S3A, Supporting Information). High resolution Nb 3d XPS spectra indicate that the peaks centered at 207.1 and 209.7 eV are attributed to the binding energy of Nb 3d 5/2 and Nb 3d 3/2 electrons, respectively (Figure S3B, Supporting Information).18,24 High resolution O 1s XPS spectra (Figure S3C, Supporting Information) show that a main peak centered at 530.3 eV is ascribed to the oxygen anions (O2−) bound to the niobium in the lattice for both Nb3O7(OH) and Nb2O5.18,24 However, an additional peak at 532.9 eV can be clearly observed for the Nb3O7(OH) sample, indicating the presence of OH groups.18,24,28 Also, similar Raman spectra obtained from these two samples imply insignificant changes in crystal structures (Figure S4, Supporting Information), indicating their structural similarity. This explains why the Nb3O7(OH) nanowire superstructures can be readily transformed into monoclinic Nb2O5 nanowire superstructures by low-temperature treatment while retaining their structural characteristics. N2 adsorption−desorption isotherm displays typical type IV curves (type H3 hysteresis loop) for both Nb3O7(OH) and Nb2O5 samples.29 The obtained type H3 hysteresis loops that do not level off at relative pressures close to the saturation vapor pressure result from the slit-like pore structures of nanowires.29 The results 509

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planes of HNbO3 and Nb3O7(OH) enable the formation of 3D Nb3O7(OH) superstructures assembled with a single crystalline nanowire array (Figure 2D). The above-discussed 3D Nb3O7(OH) nanowire superstructure formation pathway differs remarkably from those reported formation mechanisms.20,26 Monoclinic Nb2O5 has been widely investigated for use in a lithium intercalation reaction (xLi+ + xe− + Nb2O5 → LixNb2O5) because of its great potential as an electrode material.18,22,30−32 In this work, the obtained 3D orthorhombic Nb3O7(OH) and monoclinic Nb2O5 nanowire superstructures were investigated and compared as LiB anode materials. Figure 3A shows the charge and discharge curves of the Nb3O7(OH)

decreases obviously with reaction time. After 6 h of hydrothermal reaction, new diffraction peaks appear, indicating the changes in crystal structures. The samples after 9 h of reaction exhibit a typical Nb3O7(OH) crystal structure (JCPDS, Card No. 31-0928). These XRD data indicate that the Nb3O7(OH) structures could be formed through structural rearrangements of the cubic HNbO3 intermediates, which is confirmed by the SEM images of the samples fabricated under different reaction times. The as-synthesized samples with reaction times of 1 h, 3 h, and 6 h display cubic-shaped structures. The sizes of the cube-shaped structures increase with increasing hydrothermal reaction time from ∼750 nm for the sample with 1 h of reaction time to ∼980 nm for the sample with 6 h of reaction time. When the reaction time was increased to 9 h, the sprout-like protuberances start to appear on the surface of the cubic-shaped structure, signifying the growth of Nb3O7(OH) single crystalline nanowires with preferential orientation. The entire surfaces of the cubic-shaped structure are covered by an evenly distributed nanowire array with further increased reaction times (i.e., 12 and 18 h). The size of the cubic-shaped structure increases from ∼1.0 μm for the sample with 9 h of reaction time to ∼2.3 μm for the sample with 18 h of reaction time. The matured 3D Nb3O7(OH) superstructures assembled with a highly ordered single crystalline nanowire array can be obtained after 24 h of hydrothermal treatment (Figure 1B, C). On the basis of the above results, the formation process of the 3D Nb 3O 7(OH) nanowire superstructures can be proposed. In the presence of the concentrated HCl, NbCl5 as reaction precursor hydrolyzes slowly to first form HNbO3 building blocks, as shown in eq 1: HCl

NbCl5 + 3H 2O ⎯⎯⎯→ HNbO3 + 5HCl

(1)

HCl

3HNbO3 ⎯⎯⎯→ Nb3O7 (OH) + H 2O

(2)

Further, the HNbO3 building blocks start to grow and form large size cubic-shaped structures. With increasing reaction time, the size of the cubic-shaped structure increases, indicating that the growth of the cubic HNbO3 structure is predominant with shorter reaction times. After 6 h, the growth of the cubicshaped structure almost approaches equilibrium status. With a further increased reaction time (e.g., 9 h), the surface of the cubic-shaped structure begins to dissolve and rearrange, and the Nb3O7(OH) crystal structure starts to form and grow (eq 2). The matured 3D Nb3O7(OH) nanowire superstructures form after 24 h of hydrothermal reaction. During the calcination process, the orthorhombic Nb3O7(OH) is transformed into the monoclinic Nb2O5 (eq 3):

Figure 3. (A) The discharge−charge curves of Nb3O7(OH) and Nb2O5 electrodes in the voltage window of 1.2−3.0 V at the current density of 20 mA g−1. (B) Rate performance and columbic efficiency of Nb3O7(OH) and Nb2O5 electrodes at different current rates.

and Nb2O5 electrodes within the voltage window of 1.2−3.0 V at the current density of 20 mA g−1. Although the Nb2O5 superstructures have a smaller surface area compared to the Nb3O7(OH) superstructures, the Nb2O5 electrode exhibits significantly higher charge and discharge specific capacities than those of the Nb3O7(OH) electrode. The initial discharge capacities for Nb3O7(OH) and Nb2O5 electrodes are 152 and 225 mA hg−1, corresponding to x = 2.3 for LixNb3O7(OH) and x = 2.2 for LixNb2O5, respectively.22,30−32 After two cycles, the discharge capacity of the Nb2O5 electrode still remains relatively stable, reaching a high capacity of 203 mA h g−1, while the Nb3O7(OH) electrode displays a noticeable decrease in the discharge capacity (55 mA h g−1), suggesting a superior cycling stability of the Nb2O5 electrode over the Nb3O7(OH) electrode. The rate performance and columbic efficiency of the Nb3O7(OH) and Nb2O5 electrodes at different current densities demonstrate that the Nb2O5 electrode possesses higher specific capacity for LiBs and better reversible charge−

450 ° C

2Nb3O7 (OH) ⎯⎯⎯⎯⎯→ 3Nb2 O5 + H 2O

(3)

The theoretical calculations were performed in this work to validate the proposed reaction pathway. Our results indicate that the transformation of cubic HNbO3 to orthorhombic Nb3O7(OH) is thermodynamically favorable with an exothermic energy of −3.98 eV (Figure 2C), confirming the preferential formation of 3D Nb3O7(OH) nanowire superstructure. The calculation results also indicate that the cubic HNbO3 structure possesses six equivalent (100) planes (d value of 0.39 nm). With a sufficient reaction time (e.g., 9 h), the preferential structural growth perpendicular to the six equivalent (100) planes leads to surface dissolution to form the Nb3O7(OH) crystal structure. The well matched (100) 510

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ACS Nano discharge cycle capability (Figure 3B). The cycle performance of the Nb2O5 electrode obtained at a constant charge/discharge current density of 100 mA g−1 further demonstrates the high capacity and excellent cycling stability of the Nb2O5 electrode (177 mA h g−1 after 150 cycles, Figure S6 in Supporting Information). The above results show that the Nb3O7(OH) and Nb2O5 electrodes have a very similar lithium intercalation number (x = 2.3 for LixNb3O7(OH) and x = 2.2 for LixNb2O5), which can be attributed to their structural similarity. The atomic structures of Nb3O7(OH) and Nb2O5 (Figure S7, Supporting Information) show very similar channel structures for both materials, which is beneficial to lithium intercalation.33−35 However, an important difference between these two materials is the presence of hydroxyl groups in Nb3O7(OH). Our previous theoretical calculations indicated that the most stable Nb3O7(OH) crystal structure can be configured by bonding a H atom to an O atom at the (002) plane and interacting with another nearest O atom at the same plane though a hydrogen bond to form O−H·O.24 The presence of hydroxyl groups in the Nb3O7(OH) structure could be disadvantageous for battery performance, because −OH groups in the Nb3O7(OH) structure are highly stable during charge and discharge processes, which could potentially occupy the active sites and obstruct Li+ intercalation.36−38 In addition, the large molecular weight of the Nb3O7(OH) is also unfavorable for high capacity in comparison with the Nb2O5 with relatively low molecular weight. These could be the reasons that the 3D Nb2O5 nanowire superstructures possess a relatively low surface area compare to the 3D Nb3O7(OH) nanowire superstructures but exhibit higher capacity and cycling stability as electrode material for LiBs. In our previous report, a single crystal Nb3O7(OH) nanorod film on an FTO substrate without calcination has been directly used as the photoanode for DSSCs, achieving an impressive overall efficiency of 6.77%.24 In this work, the as-synthesized 3D Nb3O7(OH) nanowire superstructure film directly grown onto a FTO substrate via a simple hydrothermal method (see experimental section for details) was also directly used as the DSSCs photoanode. XRD data demonstrate that the fabricated film possesses identical orthorhombic Nb3O7(OH) crystal structures as those shown in Figure 1 (Figure S8, Supporting Information). The typical surface SEM image shows that the formed Nb3O7(OH) nanostructured film is composed of nanowire superstructures (Figure 4A). Figure 4B shows the cross-sectional SEM image of the obtained Nb3O7(OH) nanowire superstructure film. As can be seen, the formed film has a thickness of ∼17.5 μm. Interestingly, a highly ordered nanowire layer directly grown from the FTO substrate can be clearly observed between the nanowire superstructure layer and the FTO substrate (Figure 4C and D). This highly ordered Nb3O7(OH) nanowire layer has a thickness of ∼580 nm, formed by nanowires having uniform diameters around 23 nm. Such a nanowire layer could play an important role in connecting Nb3O7(OH) nanowire superstructures and the FTO substrate to form a highly stable film structure. As DSSCs photoanode, the nanowire layer can also act as an effective blocking layer to prevent photoelectron leakage at the interface between the nanowire superstructure layer and FTO substrate, thus improving DSSCs efficiency.39,40 Additionally, the highly ordered single crystalline nanowires are also beneficial for facilitating the photoelectron transport inside photoanode film to enhance DSSCs performance.41,42

Figure 4. (A) Surface SEM image of as-synthesized Nb3O7(OH) film onto the FTO substrate. (B) Low-magnification and (C) highmagnification cross-sectional SEM images of as-synthesized Nb3O7(OH) film onto the FTO substrate. (D) Cross-sectional SEM image of Nb3O7(OH) single crystalline nanowire layer.

Without the need for further calcination, the as-synthesized Nb3O7(OH) nanowire superstructure film on a fluorine-doped tin oxide (FTO) substrate was directly used as a DSSC photoanode. For comparison, the Nb2O5 film photoanode was fabricated by the thermal treatment of Nb3O7(OH) film at 450 °C. The DSSCs assembled by both photoanodes were sensitized with N719 dye and subjected to performance measurement under the standard AM 1.5 simulated sunlight (100 mW cm−2). Figure 5 shows photocurrent−photovoltage

Figure 5. Photocurrent as a function of photovoltage for DSSCs assembled with Nb3O7(OH) and Nb2O5 film photoanodes.

curves (J−V curves) of the assembled DSSCs. The DSSCs assembled with Nb3O7(OH) photoanode exhibits an overall efficiency of 6.38% with a short-circuit current density (Jsc) of 12.76 mA cm−2, an open-circuit voltage (Voc) of 731 mV, and a fill factor (FF) of 68.4%. Comparatively, the DSSCs assembled with a Nb2O5 photoanode possess slightly higher Voc (740 mV) and FF (68.7%) but much lower Jsc (11.55 mA cm−2), resulting in a lower overall efficiency (5.87%), which is due mainly to the decreased surface area and dye loading amount (1.71 × 10−7 and 1.53 × 10−7 mol cm−2 for Nb3O7(OH) and Nb2O5 photoanodes, respectively) (Figure 5). A number of studies 511

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samples as electrode materials were subsequently evaluated in lithiumion batteries and dye-sensitized solar cells. Characterization. SEM (JSM-7001), TEM (Philips F20), and XRD (Shimadzu XRD-6000 diffractometer) were employed for characterizing the prepared samples. Chemical compositions of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, Kratos Axis ULTRA incorporating a 165 mm hemispherical electron energy analyzer). Nitrogen sorption−desorption isotherms of the samples were obtained on a Quantachrome Autosorb-1 surface area and pore size analyzer. The pore size distributions of the samples were derived from the adsorption branches of the isotherms based on the Barett−Joyner−Halenda (BJH) model. Diffuse reflectance spectra of the films were recorded on a Varian Cary 5E UV−vis−NIR spectrophotometer. Raman spectra were collected with a Renishaw 100 system Raman spectrometer using a 632.8 nm He−Ne laser. The scattered light was detected with a Peltier-cooled CCD detector with spectral resolution of 2.0 cm−1. The grating was calibrated using the 520.0 cm−1 silicon band. Theoretical Calculations. All computations are performed using the Vienna ab initio simulation package (VASP) based on the allelectron projected augmented wave (PAW) method.46,47 A plane-wave basis set is employed to expand the smooth part of wave functions with a kinetic energy cutoff of 520 eV. For the electron−electron exchange and correlation interactions, the functional of PBE,48 a form of the general gradient approximation (GGA), is used throughout. The Brillouin-zone integrations were performed using Monkhorst−Pack grids of special points, with gamma-point centered (2 × 8 × 8) kpoints meshes used for the bulk cells. When the geometry is optimized, all atoms are allowed to relax. A variable cell technique is employed to simultaneously optimize the lattice constant and the atomic structure to encounter the complexity and low-symmetry of the HNbO3 and Nb3O7(OH) crystals. The lattice constants and geometric structure were optimized until the residual forces were below 0.001 eV/Å. Measurements. Lithium-Ion Batteries (LiBs). To test the electrochemical performance, the as-prepared sample was mixed at a ratio of 80 wt % active materials (Nb3O7(OH) and Nb2O5) with 10 wt % carbon black and 10 wt % binder, poly(vinylidene fluoride) (PVDF). The slurry made by using N-methyl-2-pyrrolidone (NMP) as the solvent was uniformly pasted onto pieces of Al foil with an area of 1.0 cm2. Such prepared electrode sheets were dried at 100 °C in a vacuum oven for 12 h. Then, the electrodes were compressed before making the cells. The electrochemical cells (CR 2032 coin-type cells) that were prepared for testing included the as-prepared sample on Al foil as the working electrode, Li foil as the counter and reference electrode, a porous polypropylene film as the separator, and 1.0 M LiPF6 (battery grade 99.99%, Aldrich) in a 1:2 (v/v) mixture of ethylene carbonate (EC, anhydrous 99%, Sigma-Aldrich) and diethyl carbonate (DEC, anhydrous 99+%, Sigma-Aldrich) as the electrolyte. The cells were assembled in an Ar-filled glovebox. The cells were cycled at a current density of 20 mA g−1 for the first five cycles and then cycled at different current densities for the following cycles between 1.2 and 3.0 V using a computer-controlled charger system manufactured by Land Battery Testers. The typical electrode weight was approximately 3.0 mg cm−2. Dye-Sensitized Solar Cells (DSSCs). Prior to DSSCs measurements, all niobium oxide-based photoanodes were sensitized with dye N719 (3 × 10−4 mol L−1) for 24 h. A series of DSSCs were fabricated with a traditional sandwich-type configuration by using a dye-anchored niobium oxide-based film and a platinum counter electrode deposited on FTO conducting glass. A mask with a window area of 0.15 cm2 was applied on the Nb3O7(OH) and Nb2O5 photoanode film side to define the active area of the cells. A 500 W Xe lamp (Trusttech Co., Beijing) with an AM 1.5G filter (Sciencetech, Canada) was used as the light source. The light intensity was measured by a radiant power meter (Newport, 70260) coupled with a broadband probe (Newport, 70268). The photovoltaic measurements of the DSSCs were recorded by a scanning potentiostat (model 362, Princeton Applied Research, US). The IPCE as a function of wavelength was measured with a QE/ IPCE measurement kit (NewSpec).

have demonstrated that Jsc can be markedly enhanced by improving the light utilization efficiency of a photoanode.43,44 Diffuse reflectance spectra of the Nb3O7(OH) and Nb2O5 films with similar thickness indicate that both films possess high light reflection ability over the entire visible and near-infrared ranges, demonstrating a superior light scattering property (Figure S9A, Supporting Information).43−45 The superior light scattering ability of photoanode films can be ascribed to the large sizes of cubic-shaped structures.43−45 Compared to the Nb3O7(OH) film, the Nb2O5 film has a higher light scattering ability due possibly to the formation of larger size aggregates of cubicshaped structure within the thermally treated film, giving a rise to higher light utilization efficiency. However, the lower overall efficiency obtained from the DSSCs assembled with Nb2O5 photoanode suggests that the decreased surface area is the dominant factor responsible for the decreased Jsc that leads to the decreased DSSCs performance, which is supported by the measured incident photon to current conversion efficiency (IPCE) spectra (Figure S9B, Supporting Information).

CONCLUSIONS In summary, highly ordered single crystalline nanowire array constructed Nb3O7(OH) superstructures with high a surface area of 121.5 m2 g−1 have been successfully fabricated by a simple hydrothermal method. Owing to their structural similarity, the orthorhombic Nb3O7(OH) nanowire superstructures can be readily transformed into monoclinic Nb2O5 nanowire superstructures with a surface area of 81.8 m2 g−1 by low-temperature calcination at 450 °C. The obtained Nb2O5 nanowire superstructure as LiBs anode material exhibits high capacity of 177 mA h g−1 at a constant charge/discharge current density of 100 mA g−1 and excellent cycling stability. Additionally, the directly grown Nb3O7(OH) nanowire superstructure film on FTO substrate can be achieved by a facile hydrothermal method and directly used as a DSSC photoanode, displaying an overall efficiency of 6.38%. The findings of this work demonstrate the feasibility of utilizing 3D niobiumbased nanowire superstructures as a promising means to improve the materials’ performance for energy storage and conversion applications. METHODS Synthesis. 3D Nb3O7(OH) superstructures assembled with highly ordered single crystalline nanowire array were synthesized by a similar method reported in our previous work.24 In a typical synthesis, 0.5403 g of niobium(V) chloride (NbCl5, Aldrich) was first dissolved in 40 mL of 6.6 M hydrochloric acid (HCl, 32%, Sigma-Aldrich) solution with a NbCl5 solution concentration of 0.05 M. After stirring for 2 min, the resulting solution was transferred into a Teflon-lined stainless steel autoclave with a volume of 100 mL. The hydrothermal reaction was kept at 200 °C for 24 h. After hydrothermal reaction, the autoclave was cooled to room temperature, and the as-synthesized product was adequately washed with deionized water and collected by centrifugation. The fabricated product was allowed to dry in a nitrogen stream for further characterization and use. For the fabrication of Nb3O7(OH) film, a piece of pretreated FTO conducting glass (15 Ω/square, Nippon Sheet Glass, Japan) with the conductive side facing up was first immersed into the precursor solution prior to hydrothermal reaction. The hydrothermal reaction was performed under the same experimental conditions as the aforementioned procedure. After the hydrothermal reaction, the FTO substrate was taken out, rinsed adequately with deionized water, and dried in a nitrogen stream. The Nb2O5 nanostructures (films) were obtained by thermal conversion of the as-synthesized Nb3O7(OH) nanostructures (films) at 450 °C for 2 h. The resultant Nb3O7(OH) and Nb2O5 512

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b05441. Supported images, XPS, Raman, UV−vis diffuse reflectance spectra of Nb3O7(OH) and Nb2O5 and some theoretical calculation results (PDF)

AUTHOR INFORMATION Corresponding Author

*Address correspondence to h.zhao@griffith.edu.au. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the CAS Pioneer Hundred Talents Program and the Natural Science Foundation of China (Grant No. 51472246 and No. 51072199), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09030200), and the CAS/SAFEA International Partnership Program for Creative Research Teams of Chinese Academy of Sciences, China. REFERENCES (1) Whitesides, G. M.; Grzybowski, B. Self-Assembly at All Scales. Science 2002, 295, 2418−2421. (2) Gao, Y.; Tang, Z. Y. Design and Application of Inorganic Nanoparticle Superstructures: Current Status and Future Challenges. Small 2011, 7, 2133−2146. (3) Jiang, J.; Li, Y.; Liu, J.; Huang, X.; Yuan, C.; Lou, X. W. Recent Advances in Metal Oxide-Based Electrode Architecture Design for Electrochemical Energy Storage. Adv. Mater. 2012, 24, 5166−5180. (4) Dong, Z.; Lai, X.; Halpert, J. E.; Yang, N.; Yi, L.; Zhai, J.; Wang, D.; Tang, Z.; Jiang, L. Accurate Control of Multishelled ZnO Hollow Microspheres for Dye-Sensitized Solar Cells with High Efficiency. Adv. Mater. 2012, 24, 1046−1049. (5) Wang, J.; Yang, N.; Tang, H.; Dong, Z.; Jin, Q.; Yang, M.; Kisailus, D.; Zhao, H.; Tang, Z.; Wang, D. Accurate Control of Multishelled Co3O4 Hollow Microspheres as High-Performance Anode Materials in Lithium-Ion Batteries. Angew. Chem., Int. Ed. 2013, 52, 6417−6420. (6) Pan, A.; Wu, H. B.; Yu, L.; Lou, X. W. Template-Free Synthesis of VO2 Hollow Microspheres with Various Interiors and Their Conversion into V2O5 for Lithium-Ion Batteries. Angew. Chem., Int. Ed. 2013, 52, 2226−2230. (7) Dong, Z. H.; Ren, H.; Hessel, C. M.; Wang, J. Y.; Yu, R. B.; Jin, Q.; Yang, M.; Hu, Z. D.; Chen, Y. F.; Tang, Z. Y.; et al. QuintupleShelled SnO2 Hollow Microspheres with Superior Light Scattering for High-Performance Dye-Sensitized Solar Cells. Adv. Mater. 2014, 26, 905−909. (8) Xu, S. M.; Hessel, C. M.; Ren, H.; Yu, R. B.; Jin, Q.; Yang, M.; Zhao, H. J.; Wang, D. Alpha-Fe2O3 Multi-Shelled Hollow Microspheres for Lithium Ion Battery Anodes with Superior Capacity and Charge Retention. Energy Environ. Sci. 2013, 7, 632−637. (9) Ji, L.; Lin, Z.; Alcoutlabi, M.; Zhang, X. Recent Developments in Nanostructured Anode Materials for Rechargeable Lithium-Ion Batteries. Energy Environ. Sci. 2011, 4, 2682−2699. (10) Li, G. L.; Liu, J. Y.; Lan, J.; Li, G.; Chen, Q. W.; Jiang, G. B. 3D Hierarchical Anatase TiO2 Superstructures Constructed by ″Nanobricks″ Built Nanosheets with Exposed {001} Facets: Facile Synthesis, Formation Mechanism and Superior Photocatalytic Activity. CrystEngComm 2014, 16, 10547−10552. (11) Heiniger, L. P.; Giordano, F.; Moehl, T.; Gratzel, M. Mesoporous TiO2 Beads Offer Improved Mass Transport for 513

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